Mr Willie Thompson, the straight-talking MP for West Tyrone, finds himself at odds with his UUP constituency officers and local councillors, almost all of whom have publicly declared themselves to be in the Yes camp.
Some party members say Mr Thompson's calls for Mr David Trimble to resign, and his appearances at No campaign meetings with DUP leaders, have alienated UUP supporters in the constituency.
Mr Thompson won the seat in last year's Westminster general election because the DUP agreed not to run a candidate, and the nationalist vote was split by Sinn Fein and the SDLP.
While UUP constituency association members currently rule out any moves to oust their rebellious MP from the party, he is well aware that he could face that fate in the months ahead. If expelled from the party, Mr Thompson would remain an independent unionist MP.
Mr Thompson (58), a local businessman and Methodist lay preacher, claims to have the support of between 60 and 70 per cent of the area's DUP and UUP voters combined. But his party's Yes campaigners say that about 60 per cent of UUP voters in the constituency do not support his stance on the agreement.
"A lot of people are disappointed with his stance. People feel he has been harmful to the party," said a Strabane councillor, Mr Jim Emery, a Yes voter and campaign organiser for the West Tyrone Constituency Association.
Mr Emery said Mr Trimble received a warm welcome on the streets of Omagh last week when he was promoting the Yes campaign. "People were coming over and wishing us all the best and shaking hands with Mr Trimble. It was very heartening," he said.
Mr Wilfred Breen, a retired teacher and former chairman of Omagh District Council, said he supports Mr Thompson, but disagreed with his calls for Mr Trimble to resign as party leader. "I might be on the losing side of the party, but we're democrats. It's a matter of conscience and we shouldn't be brow-beaten," he said.
Mr Crawford McFarland, the chairman of the West Tyrone Constituency Association, who is also supporting the Yes campaign, described the mood among many local Ulster Unionists as "uneasy".
"Some people feel uncomfortable with the situation and some just don't know what to do," he said,
Mr McFarland, a local dairy farmer and an Omagh councillor for 31 years, is clearly ill at ease to find himself lined up against his local MP who, he said, he deeply respects. A quiet and mannerly man, he would not criticise Mr Thompson, and said the constituency association which selected him to run in last year's general election "wouldn't be of a mind to expel or sack him".
"I would be hoping that after the 22nd is over and if it's a Yes vote, and we'll have to get into the position of an election, Willie would come along and make his contribution to help and support it," he said.
Mr Thompson, however, said he was not "terribly interested" in standing for a future assembly and would continue to support those opposed to the agreement. "I wouldn't leave the party, but I wouldn't support a Yes candidate in the assembly election, and if that means the party leaving me and me ceasing to be an Ulster Unionist Party MP, then that's a matter for them."
Mr McFarland said the party would probably field two candidates in the assembly elections. Party sources say that Strabane councillors, Mr Derek Hussey and Mr Edward Turner, currently appear to be the most likely candidates.
Mr John Hunter, the secretary of the Omagh Unionist Association, said the constituency officers' stance in favour of the agreement did not represent the party's rank and file.